Acrylic polymer sheet such as PLEXIGLAS polymethyl methacrylate (PMMA) sheet (Arkema Inc.) is an optically clear plastic having superior optical clarity and excellent weathering performance as well as a lower specific gravity than glass, making PMMA a lighter weight material. Because of its excellent properties and lower weight PMMA is often used as a replacement for glass.
Photovoltaic (PV) modules generally have low iron tempered glass front panels, due to a need for high light transmittance (>90.5%) and excellent weathering performance. However, the glass front panels are generally heavy (bulk density ˜2.5 g/cm3) and fragile.
The use of acrylic sheet as glazing for a photovoltaic module is described in WO 08/19229, including the use of PMMA sheet having a polyvinylidene fluoride coating for dirt-shedding. Unfortunately, issues can arise from the use of standard PMMA sheet in a photovoltaic module.
Generally, PV modules are manufactured by a lamination process in which many or most of the separate components are heated together at a temperature in the range of 130° C. to 150° C. for up to 45 minutes to enable crosslinking of an encapsulate, and seating of the other components. In use, photovoltaic modules have been reported to experience continuous temperatures of between −10° C. and 130° C. Standard acrylic sheets have amaximum recommended continuous service temperature of between 63° C. and 102° C.
Specialty acrylic sheets can be produced having maximum continuous service temperatures of up to 115° C. and even higher temperatures for shorter periods of time, without significant deformation.
U.S. Pat. No. 6,653,405 describes a blend of PMMA with a poly(methylmethacrylate-co-t-butyl methacrylate) copolymer to produce a high Tg acrylic with good clarity and low haze, useful for coverings on lamps.
Higher Tg acrylic copolymers have been used for outdoor applications, including in PV concentrator systems (Mahoney et al. “Accelerated UV-aging of Acrylic Materials used in PV Concentrator Systems” Sandia National Laboratories, Albuquerque, N. Mex.). The reference describes Plexiglas® P55, a methylmethacrylate-methacrylamide copolymer with 4.8% methacrylamide, and having a Tg of approximately 122° C. as measured by DSC. The other acrylic materials (V811, Plexiglas® K, and Implex) described in the reference have Tg's≤105° C. Unfortunately, the poor solubility of methacrylamide in methylmethacrylate limits the methacrylamide level to 5 wt % in a methylmethacrylate-methacrylamide copolymer, thereby limiting the maximum achievable glass transition temperature to approximately 122° C. as measured by DSC. The methylmethacrylate-methacrylimide copolymer also tends to yellow under certain conditions.
Applicant has found that high Tg acrylic copolymer sheet can be used as a front glazing on a photovoltaic module, providing higher performance and good weathering without significant deformation of the acrylic sheet. Useful acrylic copolymers contain no acrylamide or methacrylamide monomer units, while terpolymers may contain up to 3 weight percent acrylamide or methacrylamide monomer units.